In recent years, along the distribution of various multimedia such as Internet, there has been an increasing demand for the increase in the capacity of optical communications. Conventionally, in the optical communications, it has been general to transmit information using a single wavelength in a band of 1310 nm or 1550 nm as a wavelength that has small light absorption by an optical fiber. In this method, it is necessary to increase the number of cores of optical fibers that are laid in a transmission path in order to transmit a large quantity of information. Therefore, there has been a problem of increase in cost along the increase in the transmission capacity.
Therefore, a DWDM (Dense-Wavelength Division Multiplexing) communication method has come to be used. In this DWDM communication method, an EDFA (Erbium Doped Fiber Amplifier) is mainly used, to transmit information using a plurality of wavelengths in the 1550 nm band as the operation band thereof. In the DWDM communication method or the WDM communication method, optical signals of a plurality of different wavelengths are simultaneously transmitted using one optical fiber. Therefore, it is unnecessary to newly add a line, and thus, it is possible to remarkably increase the transmission capacity of the network.
A general WDM communication method that uses the EDFA has been put into practical use from 1550 nm which is easy for flatting the gain, and recently, the band has been expanded to 1580 nm which has not been utilized due to a small gain coefficient. However, as a low-loss band of the optical fiber is wider than a band capable of being amplified by the EDFA, there has been high interest in an optical amplifier operated in the band outside the EDFA band, i.e., the Raman amplifier.
A gain wavelength range of an optical amplifier using a rare earth ion such as erbium as a medium is determined by an energy level of ion. On the other hand, the Raman amplifier has a characteristic that the gain wavelength range is determined by a wavelength of an exciting light. Therefore, it is possible to amplify any optional wavelength range by selecting the exciting light wavelength.
In the Raman amplification, when a strong exciting light is incident to the optical fiber, a gain appears on the long wavelength side by about 100 nm from the exciting light wavelength by a stimulated Raman scattering. When a signal light in the wavelength range having this gain is incident to the optical fiber in this excited state, this signal light is amplified. Therefore, in the WDM communication method using the Raman amplifier, it is possible to further increase the number of channels of the signal light as compared with a communication method using the EDFA.
FIG. 22 is a block diagram that shows a structure of a conventional Raman amplifier that is used for the WDM communication system. In FIG. 22, semiconductor laser modules 182a to 182d which include Fabry-Perot type semiconductor light emission elements 180a to 180d and fiber gratings 181a to 181d in pairs respectively, output laser beams which are the excitation light source to polarization beam combiners 61a and 61b. The wavelengths of laser beams output from the respective semiconductor laser modules 182a and 182b are the same, but the light having a different planes of polarization is multipexed by the polarization beam combiner 61a. Similarly, the wavelengths of laser beams output from the respective semiconductor laser modules 182c and 182d are the same, but light having different planes of polarization is multipexed by the polarization beam combiner 61b. The polarization beam combiners 61a and 61b output the polarization-multipexed laser beams respectively to the WDM coupler 62. The wavelengths of laser beams output from the polarization beam combiners 61a and 61b are different from each other.
The WDM coupler 62 couples the laser beams output from the polarization beam combiners 61a and 61b through an isolator 60, and outputs it to an amplification fiber 64 as the exciting light through a WDM coupler 65. The signal light to be amplified is input from a signal light input fiber 69 through an isolator 63 to the amplification fiber 64 to which the exciting light has been input, and it is coupled with the exciting light and is Raman-amplified.
The signal light (amplified signal light) Raman-amplified in the amplification fiber 64 is input to a monitor light distribution coupler 67 through a WDM coupler 65 and an isolator 66. The monitor light distribution coupler 67 outputs a part of the amplified signal light to a control circuit 68, and outputs the remaining amplified signal light to a signal optical output fiber 70 as an output laser beam.
The control circuit 68 controls the light emitting state, e.g., the optical intensity, of the respective semiconductor light emission elements 180a to 180d based on a part of the input amplified signal light, and performs feedback control to obtain a flat characteristic in the gain band of the Raman amplification.
FIG. 23 is a diagram that shows a schematic structure of the semiconductor laser module that uses the fiber grating. In FIG. 23, a semiconductor laser module 201 has a semiconductor light emission element 202 and an optical fiber 203. The semiconductor light emission element 202 has an active layer 221. The active layer 221 is provided with a light reflection surface 222 at one end, and is provided with a light radiation surface 223 at the other end. Light generated in the active layer 221 is reflected by the light reflection surface 222, and is output from the light radiation surface 223.
The optical fiber 203 is disposed on the light radiation surface 223 of the semiconductor light emission element 202, and is optically coupled to the light radiation surface 223. In a core 232 in the optical fiber 203, a fiber grating 233 is formed at a predetermined position from the light radiation surface 223, and the fiber grating 233 selectively reflects light of the specific wavelength. That is, the fiber grating 233 functions as an external resonator, and forms a resonator between the fiber grating 233 and the light reflection surface 222. A laser beam of a specific wavelength selected by the fiber grating 233 is output as an output laser beam 241.
However, in the above-described semiconductor laser module 201 (182a to 182d), a distance between the fiber grating 233 and the semiconductor light emission element 202 is long. Therefore, RIN (Relative Intensity Noise) becomes large due to resonance between the fiber grating 233 and the light reflection surface 222. In the Raman amplification, the process in which the amplification occurs comes early. Therefore, when the exciting light intensity is fluctuated, the Raman gain also fluctuates. The fluctuation of the Raman gain is directly output as the fluctuation of the amplified signal intensity, which causes a problem in that stable Raman amplification cannot be carried out.
For the Raman amplifier, there are also a front-side excitation method which excites a signal light from the front-side, and a bi-directional excitation method which bi-directionally excites a signal light, in addition to a rear-side excitation method which excites a signal light from the rear side, like the Raman amplifier shown in FIG. 32. At present, the rear-side excitation method is mainly used as the Raman amplifier. The reason is that the front-side excitation method in which the weak signal light proceeds in the same direction together with the strong exciting light has a problem in that the exciting light intensity fluctuates in the semiconductor laser module using the fiber grating. Therefore, a stable excitation light source that can be applied to the front-side excitation method has been required. That is, there has been a problem that the applicable excitation method is limited, when a semiconductor laser module using the conventional fiber grating is used.
Further, the semiconductor laser module 201 needs to optically couple the optical fiber 203 having the fiber grating 233 with the semiconductor light emission element 202. As the optical coupling is carried out mechanically in the resonator, there is a risk that the oscillation characteristic of the laser may vary due to mechanical vibrations. As a result, there has been a problem that it is not possible to provide stable exciting light.
Further, the Raman amplification in the Raman amplifier is based on a condition that a polarization direction of the signal light and a polarization direction of the exciting light coincide with each other. That is, the Raman amplification has a polarization dependency of the amplified gain, and it is necessary to reduce an influence caused by a deviation between the polarization direction of the signal light and the polarization direction of the exciting light. According to the rear-side excitation method, the signal light has no problem as the polarization becomes random during propagation. However, according to the front-side excitation method, the polarization dependency is strong, and it is necessary to reduce the polarization dependency by cross polarization synthesis, or depolarization or the like of the exciting light. That is, it is necessary to reduce the degree of polarization (DOP).